Molding element comprising cutting means for molding and vulcanizing a tire tread

ABSTRACT

A molding element of a mold for molding and vulcanizing a tire tread, this tread having a tread surface intended to come into contact with the ground when the tire is rolling. This molding element has a molding surface adapted to mold part of the tire tread surface and a blade adapted to mold a sipe or a groove in the tread. The molding element has two lamella positioned on either side of the blade at a distance from this blade, each lamella having a bulge at one extremity far away from the molding surface. Each lamella having cutting means extending from the bulge.

This application is a 371 national phase entry of PCT/EP2014/061802,filed 6 Jun. 2014, which claims benefit of European Patent ApplicationNo. 13172040.1, filed 14 Jun. 2013, the contents of which areincorporated herein by reference for all purposes.

BACKGROUND

The present disclosure relates to the field of molds for molding andvulcanizing a tire tread. More specifically, the disclosure relates tothe molds used for molding in the tread grooves which are partially orcompletely covered by an additional cover layer.

DESCRIPTION OF RELATED ART

It is known practice to design tires the tread of which comprisesvarious rubber compounds. Document WO 03089257 discloses such treads.More specifically, document WO 03089257 discloses a tread comprisinggrooves the walls of which are covered with a cover layer. The materialof which this cover layer is made differs from the rubber compound ofwhich the tread is made. This material notably has much better wet gripthan the wet grip of the rubber compound. This allows a very significantimprovement in cornering performance on wet ground.

One way of manufacturing this tread is notably disclosed in document WO2006069912. According to this method of manufacture, in a first step,provision is made for the material intended to constitute a cover layerto be injected in the form of one or more inserts into the green tireusing an injection nozzle. The insert or inserts is or are then shaped,in a second step, by ribs of a vulcanizing mold so that they cover allor part of the walls of the grooves molded by these ribs.

This method of manufacture has its limits, particularly as far asobtaining precision moldings is concerned. Specifically, during theshaping thereof, the insert experiences a significant shear force fromthe rib in order to convert this insert into a layer of lesserthickness. This shear force may cause cracking within the insert, makingit more difficult to control the movements of the material of which thisinsert is made. The shape and the thickness of the cover layer thusformed may therefore be somewhat haphazard. The advantages afforded bythe additional layer to the performance of the tire are then reduced.

In addition, in this method of manufacture, it is necessary to make theinserts align with the ribs. This makes manufacture of the tread morecomplicated.

There is therefore a need to improve the incorporation of a cover layeron the walls of grooves of a tire tread.

DEFINITIONS

A “tire” means all types of elastic tire whether or not subjected to aninternal pressure.

A “green tire” or “green form” of a tire means a superposition of aplurality of semi-finished rubber products present in the form of stripsor sheets, with or without reinforcement. The green tire is intended tobe vulcanized in a mold in order to obtain the tire.

The “tread” of a tire means a quantity of rubber material bounded bylateral surfaces and by two main surfaces one of which is intended tocome into contact with a road surface when the tire is running.

A “tread surface” means the surface formed by those points on the tiretread that come into contact with the road surface when the tire isrunning.

A “mold” means a collection of separate molding elements which, whenbrought closer towards one another, delimit a toroidal molding space.

A “molding element” of a mold means part of a mold. A molding elementis, for example, a mold segment.

A “molding surface” of a molding element means the surface of the moldthat is intended to mold the surface of the tire tread.

A “blade” of a molding element means a protrusion projecting from themolding surface. In the blades category, a distinction is made betweensipe blades which are less than 2 mm wide and ribs which have a width of2 mm or more. Sipe blades are intended to mold sipes in the tire tread,which means cuts which at least partially close up when they fall withinthe contact patch where the tire makes contact with the ground. The ribsare intended to mold grooves in the tread, which means cuts which do notclose up when they fall within the contact patch in which the tire makescontact with the ground.

SUMMARY

The invention, in an embodiment, relates to a molding element of a moldfor molding and vulcanizing a tire tread, this tread comprising a treadsurface intended to come into contact with the ground when the tire isrolling. The molding element comprises a molding surface intended tomold part of the tire tread surface and a blade intended to mold a sipeor a groove in the tread. The molding element comprises two lamellapositioned on either side of the blade at a certain distance from thisblade, each lamella comprising a bulge at one extremity far away fromthe molding surface. Indeed, each lamella comprises cutting meansextending from the bulge.

The cutting means are able to cut a cover layer that covers a greentire. The blade itself will mold a groove in the tread and at the sametime guide part of the cover layer into the groove thus molded.

The invention, in an embodiment, proposes that the cover layer is cutand some parts of this layer are positioned within the depth of thegreen tire during one and the same movement of the molding element withrespect to the green tire.

The cutting means can create a stress concentration at cutting meansconnection area. That stress causes block chipping during tire rollingand crack initiation risk in the deep of the tread band. By applying abulge at the base of the cutting means, the inventors have discoveredthat the stress concentration is reduced during tire rolling.

In an alternative form of the embodiment, the lamella extend in adirection of extension X, and the cutting means are continuous alongthis direction of extension X. Continuous cutting means improve theoperation of cutting the cover layer.

In another alternative form of the embodiment, the cutting meanscomprise a plurality of sharped teeth.

With the lamella, there is a space between two consecutive teeth. Thisspace create a gum bridge between two cavities molded by the teeth. Thisgum bridge limits also the risk of initiating cracks.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the embodiments of the invention willemerge from the following description, given by way of non-limitingexample, with reference to the attached drawings in which:

FIG. 1 is a sectional drawing depicting schematically a molding elementaccording to the embodiments of the invention;

FIG. 2 illustrates a lamella of the molding element of FIG. 1, accordingto a first embodiment of the invention;

FIG. 3 illustrates a lamella of the molding element of FIG. 1, accordingto a second embodiment of the invention;

FIG. 4 illustrates a step of a method of manufacturing using the moldingelement of FIG. 1, in which a cover layer is cut by cutting meansbelonging to the lamella;

FIG. 5 illustrates a step of the method of manufacturing using themolding element of FIG. 1, in which a blade drives a cut part of thecover layer into the green tire;

FIG. 6 illustrates a step of the method of manufacturing using themolding element of FIG. 1, in which blade and lamella are totallyembedded in the green tire;

FIG. 7 is a sectional drawing depicting schematically a tire tread witha raised element partially covered by a cover layer, after using themolding element of FIG. 1.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In the description that follows, elements that are substantiallyidentical or similar will be denoted by identical references.

FIG. 1 depicts a molding element 1 for implementing a method ofmanufacture according to the embodiment of the invention.

More specifically, the molding element 1 comprises a molding surface 3intended to mold part of the tread surface of a tire. The moldingelement 1 also comprises blades 5 of which just one is depicted here tomake the embodiment of the invention easier to understand. The blade inthis instance is a rib 5 intended to mold a groove in the tire tread. A“groove” in a tread means a cut in this tread of a width, i.e. thedistance separating two lateral walls of this groove, greater than 2 mm.The rib 5 extends heightwise and projects from this molding surface 3with an height H. The rib 5 also extends lengthwise in a direction ofextension X. In the mold, this direction X may be a circumferentialdirection following the circumference of the mold. As an alternative,the direction of extension X is a transverse direction perpendicular tothe circumferential direction of the mold. In another alternative form,this direction of extension is an oblique direction making a non-zeroangle with the circumferential direction and with the transversedirection of the mold.

The molding element 1 of FIG. 1 also has two lamella 7 arranged eitherside of the rib 5. These lamella extend lengthwise in a directionparallel to the direction of extension X of the rib 5. A “paralleldirection” means that the direction of extension of the cutting meansmakes an angle of between −5° and +5° with the direction of extension Xof the rib. Each lamella comprises a bulge 9 at one extremity far awayfrom the molding surface 3 and each lamella comprises cutting means 11extending from the bulge.

FIG. 2 illustrates lamella 7 comprising cutting means 11 continuouslyextending along the direction of extension X.

FIG. 3 illustrates another embodiment of lamella 7. In this embodiment,the cutting means 11 comprise a plurality of sharped teeth 13.

FIG. 4 to FIG. 7 illustrates the different steps of a method ofmanufacturing a tire tread using a molding element according to theembodiment of the invention.

FIG. 4 discloses a step in which the cover layer 12 is cut by thelamella 7. In this step, the molding element 1 and the green tire 10move closer to one another. This movement is, for example, initiated bya membrane (not depicted) in the mold. Under the action of a quantity ofpressurized steam, this membrane swells and pushes the green tiretowards the molding element 1.

FIG. 5 illustrates a step whereby the blade 5 drives part 14 of thecover layer into the green tire.

FIG. 6 illustrates an intermediate molding step in which the rib 5 ispushed into the green tire over its entire height. The entirety of thecover layer thus finds itself inside the green tire. Once this step hasbeen performed, it is then possible to vulcanize the green tire, i.e. toconvert the rubber material of which the green tire is made from theplastic state to the elastic state. This vulcanizing step may alsomodify the internal structure of the cover layer.

FIG. 7 depicts the result of the various steps of molding andvulcanizing the green tire illustrated in FIGS. 4 to FIG. 6. The part ofthe tread thus obtained comprises a groove 16 obtained by molding rubberaround the rib 5 and two sipes 18 obtained by molding rubber around thetwo lamella 7. It will be noted here that all of the walls of thegroove, i.e. the lateral walls and the bottom wall flanked by thelateral walls, are covered with the cut part 14 of the cover layer.

The embodiments of the invention is not restricted to the examplesdescribed and depicted and various modifications can be made theretowithout departing from its scope.

1. A molding element of a mold for molding and vulcanizing a tire tread,this tread comprising a tread surface intended to come into contact withthe ground when the tire is rolling, this molding element comprising amolding surface adapted to mold part of the tire tread surface and ablade adapted to mold a sipe or a groove in the tread, wherein themolding element has two lamella positioned on either side of the bladeat a distance from this blade, each lamella having a bulge at oneextremity far away from the molding surface and each lamella has cuttingmeans extending from the bulge.
 2. The molding element according toclaim 1, wherein each lamella extends in a direction of extension X, andthe cutting means are continuous along this direction of extension X. 3.The molding element according to claim 1, wherein the cutting means hasa plurality of sharp teeth.